1. FIELD OF THE INVENTION
This invention is related to the inhibition of corrosion of stainless steel surfaces, and more particularly to methods for inhibiting the corrosion of stainless steel surfaces which are exposed to hot alkali metal carbonate solutions in industrial processes.
2. DESCRIPTION OF THE PRIOR ART
This invention is particularly concerned with processes such as the manufacture of synthesis gas by the partial oxidation of sulfur-containing crude oil, in which the crude synthesis gas is scrubbed with carbonate solution to remove carbon dioxide together with a minor but highly significant proportion of hydrogen sulfide.
The removal of weakly acidic gases from process gas streams by the use of hot potassium carbonate solution as an aqueous absorbent solution has assumed increasing importance in recent years. Operating details of a typical process of this nature are described in U.S. Pat. No. 2,886,405. The process generally consists of a gas-scrubbing step at elevated pressure, during which the aqueous carbonate solution absorbs the weakly acidic gas or gases from the gas stream, followed by a separate regeneration step at lower pressure whereby the weakly acidic gas is removed from the liquid solution. The regenerated solution is then recycled to the gas-scrubbing step. Typical acidic gases which are removed from process gas streams in this manner include carbon dioxide, hydrogen sulfide, hydrogen cyanide and carbonyl sulfide. The potassium carbonate content of these solutions can vary within a range of about 20 to about 40 percent, depending upon the individual installation, and may contain monoethanolamine, diethanolamine, amine borates, and the like to assist gaseous absorption. In all of these compositions however, the absorbing solutions are very corrosive to ferrous metals with which they come into contact.
The process is generally carried out in carbon steel vessels, with stainless steel being employed at critical points. It has been found that chromium-nickel stainless steel, which has an austenitic metallurgical structure, while satisfactorily corrosion resistant, suffers the disadvantage of having a relatively high susceptibility to stress-corrosion cracking. For this reason chromium stainless steels, i.e., those having a martensitic or ferritic metallurgical structure, are sometimes used and corrosion of process equipment is a significant operating problem. It has heretofore been alleviated or prevented by the addition of various compounds or agents to the circulating carbonate solution. Among these may be mentioned chromates, silicates, and organic agents such as film-forming, highly polar aliphatic polyamines having two or more amino groups located at the ends of long hydrocarbon chains.
Numerous investigators have studied the corrosion of aqueous poassium carbonate solutions and the incorporation of additives in them for corrosion prevention. For example Bienstock and Field repoted in Corrosion, Vol. 17, page 337t (1961) that higher concentrations of potassium carbonate were more corrosive than lower concentrations and that sparging with carbon dioxide greatly increased this corrosion. Analysis of the solutions showed that carbon dioxide caused conversion of 15 to 20 percent of the carbonate to bicarbonate, thus suggesting that the bicarbonate is the more corrosive species. If the sparging gas contained hydrogen sulfide, corrosion was less even if carbon dioxide was present, but in commercial plant usage, operating problems still resulted from corrosion. Bienstock and Field subsequently reported in Corrosion, Vol. 17, page 571t (1961) that 0.2 percent of sodium chromate or sodium metavanadate prevented some of the attack of ferrous metals although some pitting was noted in crevices with the chromate-inhibited solutions. U.S. Pat. No. 3,181,929 covers a similar or the same proposal. These investigators also tested potassium chromate, sodium metasilicate and potassium nitrite for this purpose.
Negra and McCloskey disclosed in U.S. Pat. No. 3,087,778 that trivalent compounds of arsenic, antimony, bismuth and phosphorus acted as inhibitors for both liquid and vapor corrosive attack.
Banks, in Material Protection, Vol. 6, page 37 (1967) studied the corrosivity of used solutions from gas absorption plant installations and laboratory-prepared solutions containing concentrations of potassium carbonate and potassium bicarbonate typical of those used in service. His polarization studies indicated that metavanadate salts passivate mild steel only if the bicarbonate level was low. Once passivated, however, the steel would remain so even if the carbonate were partly converted to bicarbonate by carbon dioxide. In tests simulating plant conditions, corrosion was greater under impingement conditions such as at elbows than when film disruptive conditions were absent. Under such conditions even stainless steel alloys may also be attacked.
In other prior art in this area, U.S. Pat. No. 2,761,765 suggests that small amounts of red iron oxide (Fe.sub.2 0.sub.3) and/or alkali metal ferrites may alleviate this problem. U.S. Pat. No. 3,951,844 to Mago teaches that a mixture of a vanadium compound such as sodium metavanadate, with an antimony compound such as potassium antimonyl tartrate is effective as an anti-corrosion agent in this system. U.S. Pat. No. 3,041,135 discloses the use of petroleum sulfonates for this purpose while U.S. Pat. Nos. 3,721,526 and 3,863,003 teach the use of such anti-corrosive agents as sodium nitrite and mixtures of sodium nitrite and sodium vanadate, respectively. However, to Applicants' knowledge none of these known corrosion inhibitors have been found to be sufficiently satisfactory for their purposes to meet these problems in the use of martensitic or ferritic stainless steel materials with hot alkali metal carbonate solutions.